1. Field of the Invention
The present invention relates to a single fuel cell and a fuel cell stack. In particular, the present invention relates to a single cell and a cell stack which improve power generation performance of a PEFC (polymer electrolyte fuel cell) or PEMFC (proton exchange membrane fuel cell).
2. Description of the Related Art
The fuel cell is a power generation apparatus. In the fuel cell, a fuel gas such as hydrogen and an oxidant gas such as air are electrochemically reacted to convert chemical energy into electric energy, thereby generating power. The fuel cell has advantages such as high efficiency and a low environmental burden. Among others, because a PEFC (or PEMFC) in which a polymer is used as electrolyte can be operated at a low temperature, the PEFC (or PEMFC) is expected to be used as a household or in-vehicle power supply.
The PEFC (or PEMFC) includes a membrane electrode assembly (MEA) in which electrode catalyst layers are provided in both surfaces of a polymer electrolyte membrane. As to a method for producing the membrane electrode assembly, for example, a transfer sheet is used as a base material, catalyst ink is applied onto the transfer sheet and dried to form electrode catalyst layers, which are in turn hot-pressed on the transfer sheet, whereby the electrode catalyst layers are caused to adhere to both the surfaces of the polymer electrolyte membrane. In another method, a gas diffusion layer (GDL) is used as the base material, the catalyst ink is applied onto the gas diffusion layer to form the electrode catalyst layers, and the gas diffusion layer is hot-pressed, whereby the electrode catalyst layers are caused to adhere to both the surfaces of the polymer electrolyte membrane.
The produced membrane electrode assembly is incorporated in the single fuel cell of the PEFC (or PEMFC). In an internal structure of the single fuel cell, the gas diffusion layer is provided outside the membrane electrode assembly, and separators adhere so as to sandwich the gas diffusion layer and the membrane electrode assembly therebetween. The separator includes a gas flow channel. The separator plays a role in mainly supplying a fuel gas that is hydrogen to a fuel electrode (anode) through the gas flow channel and mainly supplying an oxidant gas that is oxygen and air to an air electrode (cathode) through the gas flow channel. Because the separator also serves as a collector that collects a current passed by an electromotive force generated in the membrane electrode assembly, the separator needs to be made of a conductive material. When the gas supplied to the anode and the gas supplied to the cathode are mixed together, an electrochemical reaction is disturbed in the electrodes, and thus it is necessary to seal the separator such that the gas supplied to the anode and the gas supplied to the cathode are not mixed together. Therefore, a gasket is disposed such that the polymer electrolyte membrane located in an outer peripheral portion of the membrane electrode assembly is covered therewith.
In the in-vehicle or household practical machine, the fuel cell refers to the stacked fuel cell in which plural single fuel cells are connected in series in order to secure large current.
In the single fuel cell, the membrane electrode assembly is opposite the gas diffusion layer. The gas diffusion layer is disposed such that the separator flow channel is covered therewith. This is because the fuel gas and oxidant gas conveyed through the separator flow channels pass through the gas diffusion layer to improve diffusivity, and therefore the fuel gas and oxidant gas easily enter the dense structure of the electrode catalyst layer.
However, occasionally the gas diffusion layer drops in the separator flow channel to cause damage, when the gas diffusion layer adheres to the separator while deviating from the separator due to a design error or a manufacturing variation. Therefore, for example, JP-A-2008-047293 discloses a fuel cell in which one of the separator flow channels is inclined with respect to the other separator flow channel in order to prevent the damage.
When a non-overlapping portion exists because the anode differs from the cathode in a shape of the gas diffusion layer, occasionally durability of the fuel cell is degraded due to a tightening load of the single fuel cell. JP-A-2007-066767 discloses a fuel cell in which the non-overlapping portion is located in a groove portion adjacent to a rib of the separator.    <Patent document 1>: JP-A-2008-047293    <Patent document 2>: JP-A-2007-066767
The fuel gas that is the hydrogen is caused to flow in the anode of the single fuel cell while the oxidant gas that is the oxygen and air is caused to flow mainly in the cathode, and the current is taken out, whereby water is generated by an electrode reaction in the cathode. The water is essential to produce proton conduction of the polymer electrolytes of the anode and cathode. However, when a large amount of water remains in the membrane electrode assembly, particularly in the cathode, the gas diffusivity is degraded to reduce fuel efficiency, which causes a problem in that power generation performance of the single fuel cell is reduced. The phenomenon is called flooding.
Further, the water generated in the cathode remains in the gas flow channel of the separator to disturb the gas supply, which causes the problem in that the power generation performance of the single fuel cell is reduced. The phenomenon is called plugging.
As described above, in the membrane electrode assembly, when the water generated by the electrode reaction exists too much or too little in the cathode catalyst layer, the power generation performance of the single fuel cell is reduced. Additionally, the problem of the flooding or plugging is generated in the anode by reverse diffusion of moisture in the gas supplied to the anode or the water from the cathode.
For example, the following measures against flooding are studied. That is, a water-repellent treatment is performed to the gas diffusion layer base material in order to improve water discharge, a mixture called MPL (Micro Porous Layer) in which water-repellent resin such as PTFE (polytetrafluoroethylene) is added to carbon particles is applied onto the electrode catalyst layer side of the gas diffusion layer, and a large amount of gas flows in the gas flow channel to discharge the water to the outside of the single fuel cell. However, unfortunately the flooding is not completely prevented, even if the water-repellent treatment is performed to the gas diffusion layer base material, or even if the MPL is provided to the gas diffusion layer. When a large amount of oxidant gas flows at one time, unfortunately cost is increased because an auxiliary machine is required, or the oxidant gas runs to waste.
As to the measure against plugging, similarly to the measure against flooding, a large amount of gas flows in the gas flow channel or the water-repellent treatment is performed to the separator flow channel. However, the cost is increased.